(1) Field of the Invention
The present invention relates to a process and apparatus for continuously leaching and extracting metal components contained in ores. More particularly, the invention relates to a process for the continuous leaching of uranium ores and other ores for recovery of metal components contained in these ores, wherein a plurality of unit layers composed of a pulverized uranium ore or other ore are laminated while a minimum necessary amount of an acid, alkali or organic solvent (hereinafter referred to as "solvent") is uniformly sprinkled on the flat surfaces of these unit layers respectively, each unit layer having a specified thickness sufficient to attain uniform mixing of the ore with the solvent, and in the state where the concentration of the solvent mixed into the ore is maintained at a high level, the heat generated by exothermic reaction caused by contact among the solvent, ore and water is effectively stored and used for thermally curing the ore, whereby the speed of extraction of the intended metal component can be increased, the leaching time shortened, the filtration characteristics improved, with the result that a highly concentrated pregnant liquor can be recovered at a high efficiency. The present invention relates also to an apparatus for practising this continuous leaching process. Throughout the specification "unit layer" means a layer having a specified thickness determined as being a sufficient thickness for attaining uniform mixing of the ore with the solvent.
(2) Brief Description of the Prior Art
For recovery of metal components contained in uranium ores and other ores by a leaching treatment, there has generally been adopted a process in which an ore is leached at a normal temperature or under heating for a long period of time by using a low concentration acid or alkali solvent. According to this conventional process, however, the speed of reaction caused by contact between the metal component in the ore and the solvent is very low. Accordingly, when a relatively refractory ore, for example, a uranium ore, is treated according to this conventional technique, the uranium recovery rate is low and further, since a long period of time is required for completion of the leaching treatment, the equipment expense becomes tremendous. Therefore, this conventional process involves many technical and economical problems which are difficult to solve. For example, when a clay uranium ore is leached with a low concentration acid or alkali according to this conventional technique, silicates contained in the ore or clay come into contact with water or dilute acid, large quantities of various colloidal substances such as SiO.sub.2 .multidot.xH.sub.2 O, having a variety of structures are dissolved out and dispersed in the leaching system, and therefore, the rate of the uranium extraction is drastically reduced by the presence of these colloidal substances. Accordingly, the leaching treatment of a clay uranium ore by the conventional technique is not only extremely difficult but also leads to great difficulties in the solid-liquid separation to be conducted after the leaching treatment. This results in a serious defect, namely a drastic decrease in the uranium recovery rate.
Various leaching processes and apparatuses have heretofore been developed and proposed as means for solving the above-mentioned various problems involved in the conventional process. For example, Japanese Pat. No. 10302/63 proposes "PROCESS AND APPARATUS FOR CONTINUOUS LEACHING OF URANIUM ORES BY THERMAL FILTRATION", in which a solvent is sprinkled on a pulverized ore continuously fed on an endless rotating belt of a given width so that a thin layer having a predetermined thickness is formed on the endless belt, and the pressure is reduced beneath the belt which is in contact with the thin layer of the pulverized ore to attain the object of uniformly mixing the pulverized ore with the solvent. In this process, however, the thickness of the layer of pulverized ore capable of being provided on the belt is limited to that which permits uniform mixing between the solvent and ore, and further, the method of sprinkling the solvent, such as sulfuric acid, involves problems. Accordingly, this process is defective in that the treating capacity per unit cannot be increased and it is impossible to increase the sulfuric acid concentration above 400 g/l in the ore-sulfuric acid admixture. Further, in "Proceedings of the Third International Conference on the Peaceful Uses of Atomic Energy", volume 12, pages 226-227, there is published a report entitled "Uranium Resources and Recovery Process in Japan", which discloses a process for treating a clay uranium ore having a high clay content. In this process, it was intended to remove water from the clay (the target water content being below 10%). However, according to this process, it is difficult to reduce the water content below 20%. Hence, sulfuric acid becomes diluted and the subsequent solid-liquid separation is difficult. Still further, in IAEA-SM-135/14 of IAEA Symposium, Sao Paulo, Aug. 1970, a pilot mill for practising the strong acid leaching process built at Ningyo-toge Mine, Okayama-ken, Japan is diagrammatically illustrated. In this apparatus, a pulverized ore and sulfuric acid fed from the upper portion of the supply end of a horizontal type puddle mixer are agitated by agitation blades rotated in the mixer, and while the mixture is moved from the supply end to the discharge end of the mixer in the agitated state (for about 15 to about 20 minutes), thermal curing of the ore is effected. Although it was intended to attain uniform mixing of the ore with sulfuric acid according to the above procedures, it was found that satisfactory results could not be obtained with this process. In addition, in IAEA-SM-135/21, there is published a report entitled "Some Recent Developments in Uranium Ore Processing Research in the United Kingdom", in which curing of uranium ores by high concentration sulfuric acid is discussed. According to the process disclosed in this report, however, satisfactory results cannot be obtained with respect to attainment of uniform mixing, and the sulfuric acid concentration in the acid-ore admixture cannot be increased over a value of 6 N. Still further, in "World Mining", May 1974, pages 40-42, a uranium processing plant at the Arlit Mill in Republic of Niger, Africa is introduced. This plant could also not sufficiently solve the problem of attainment of uniform mixing of a pulverized ore with sulfuric acid. It must also be noted that the dilution heat, described hereinafter, is not effectively utilized.
Even in the above-mentioned improved techniques, various problems such as described below are left unsolved.
When results of actual operations of leaching uranium ores with high concentration according to the improved conventional techniques are examined in detail, it is seen that an ore is pulverized to a particle size not larger than 800.mu., the amount of sulfuric acid used is about 65 Kg-H.sub.2 SO.sub.4 /ton of the ore, and the mixing ratio of sulfuric acid necessary to attain uniform mixing between the ore and sulfuric acid is such that 100 to 150 l of a sulfuric acid solution is used per ton of the ore.
When the mixing ratio is lower than 100 l/ton, the sulfuric acid concentration in the sulfuric acid solution mixed with the ore is higher than 650 g/l. Accordingly, stronger leaching with higher concentration sulfuric acid becomes possible. Further, in the case of a uranium ore containing clay or large quantities of silicates, the high concentration of sulfuric acid is advantageous in that the high concentration sulfuric acid fixes these silicates by dehydration, the solid-liquid separation after the leaching treatment being remarkably facilitated. However, uniform mixing of the ore with sulfuric acid lower than 100 l/ton becomes difficult, with resulting disadvantage that the uranium leaching rate is not increased but decreased.
When the above mixing ratio is higher than 150 l/ton, uniform mixing of the ore with sulfuric acid can be attained relatively easily, but since the sulfuric acid is diluted and the sulfuric acid concentration is reduced below 430 g/l, a long time is required for the leaching treatment and the uranium recovery rate decreases.
For the foregoing reasons, the above-mentioned mixing ratio is limited to within the range of 100 to 150 /ton in the improved conventional techniques. However, the limitation of the mixing ratio to 100 to 150 l/ton is often a great obstacle to stable continuous operation. Specifically, when the mixing ratio is in such range, in many cases the ore exhibits maximum viscosity and easily adheres to the mixer or the inner wall of the curing apparatus so that the flowing of the ore in the apparatus is inhibited. Furthermore, since the mixing ratio of100 to 150 l/ton corresponds to a sulfuric acid concentration of 650 to 430 g/l in the sulfuric acid solution, severe corrosion due to the acid takes place in the mixer or curing apparatus. Therefore, the adoption of expensive corrosion-resistant materials and the maintenance and repair of the leaching equipment becomes a heavy financial burden.
Furthermore, the majority of the heat generated in the leaching process is wasted in the improved conventional strong acid leaching process. Accordingly, the shortage of heat corresponding to the difference between the heat necessary for the thermal curing of the ore and the heat generated by dilution of sulfuric acid is made up for by preheating the ore or by using an auxiliary heat source. In short, one of the problems to be solved is that the heat generated by dilution of concentrated sulfuric acid with water contained in the ore and the heat generated by the reaction are not utilized in a sufficiently effective manner.
When an ore having a high lime content is leached with sulfuric acid, it is said that the allowable calcium carbonate content is 5 to 6% at highest. Accordingly, an alkali leaching process using sodium carbonate or the like is ordinarily adopted for ores having a high lime content. This process, however, is defective in that both the equipment capital cost and the operating cost becomes tremendous.